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UNIVERSITY OF CALIFORNIA, SAN DIEGO
SCRIPPS INSTITUTION OF OCEANOGRAPHY
" VISIBILITY LABORATORY
SAN DIEGO, CALIFORNIA 92152
AN UNDERWATER TRANSMISSOMETER FOR
OCEAN SURVEY WORK
T. J. Petzold and R. W. Austin
The subject matter of this report was presented at the Underwater Photo-Optical Instrumentation Applications Seminar sponsored by the Society of Photo-Optical Instrumentation Engineers and the Naval Undersea Warfare Center. It will be included in its present form in the published proceedings of this seminar.
SIO Ref. 68-9
April 1968
Naval Ship Systems Command Contract NObsr-95251, Task I
Project Serial No. SF0180201, Task 538
Approved: Approved for Distribution:
Seibert Q. Duntley, Director ^-* William A. Nierenberg, Director Visibility Laboratory Scripps Institution of Oceanography
AN UNDERWATER TRANSMISSOMETER FOR OCEAN SURVEY WORK
T. J. Petzold and R. W. Austin
University of California, San Diego Scripps Institution of Oceanography
Visibility Laboratory San Diego, California 92152
Abstract
A transmissometer developed at the Visibility Laboratory, University of California at San Diego is described. This instrument is capable of being operated by nontechnical persons with little instruction. No compromise has been made in the precision of measurement to obtain the design objectives which include ease of operation, low maintenance and reliability. The optical system, which has a cylindrically limited beam rather than a collimated source of light, is discussed.
Introduction
The determination of the volume attenuation coefficient, alpha, for image forming light in natural waters has become of interest to many persons working in diverse fields. It is used, along with other knowledge of the water, to compute visibility range for divers or photographic work. It is of interest to biologists and oceanographers whenever they are concerned with the radiative transfer of light flux through the water environment.
One method of determining alpha is to measure the transmission of a beam of light as it passes through a known path length of water. From this transmission measurement the attenuation coefficient, alpha, may be computed and the attenuation for any other path length may be predicted. A transmission measurement is, at first glance, an easy measurement to make. To some extent this is true; but, there are some pitfalls to be avoided if the measurement is to be truly meaningful. The usual engineering problems of working underwater must be handled; such as, watertight enclosures, ease of handling and use at sea, optical and electrical stability, etc. These problems are familiar to engineers working with oceanographic instruments.
More unique to this type of instrument is the problem of the design of the optical system. An involved system is not required but some insight is called for. To properly measure a, it is necessary to determine how much useful image forming light remains after the light has traversed a water path. This is the residual light which has been neither absorbed nor scattered by the intervening media. Thus, the detector must be allowed to receive only light which arrives undeviated from its original direction of travel, if a true alpha is to be determined. The difficulty lies in separating the image forming light from the forward non-image-forming scattered light. In fact, it is difficult, if not impossible, to achieve complete separation of the image forming light from this forward scattered light in a practical field instrument. The transmission measurement will be contaminated by the acceptance at the detector of some of the unwanted scattered flux.
Various systems employing a light source or projector and a receiver or detector separated by a water path of known length, have been used to measure beam transmittance. All of these systems accept some forward scattered light, depending primarily upon the acceptance angle, i.e., field of view of the detector.
Although it is desirable to keep this error small, the designed and user of this type of instrument must obtain a compromise between the accuracy of the alpha measurement and such factors as reliability, cost, ease of operation, short time on station, etc. The important consideration is that accuracy be commensurate with the uses to which the instrument and the data it obtains are to be put.
Design Objectives
The instrument to be described is similar in its
The development of the instrument described herein was supported in part by the U.S. Naval Ship Systems Command under Contract NObsr-95251.
1
optical concept to previous transmissometers built at the Visibility Laboratory. The approach used in the instrument's design was to take known and proven techniques and develop a reliable instrument which can be readily used by inexperienced personnel to procure accurate data under field conditions. The instrument eliminates or reduces the undesirable features of previous transmissometers. Unnecessary complexity was avoided in the interest of reducing the cost of fabrication, increasing the reliability and ease of operation all without any sacrifice of accuracy of measurement. The salient features are
1. Fixed optical alignment. 2 Temperature stability. 3. Fatigue and drift stability. 4. Insensitivity to ambient light. 5. A means of checking calibration while
instrument is submerged. 6. Operabihty by a technician with only a
short period of instruction.
Optical Design
The optical system is somewhat different from a colhmated system in that it produces a cylindri-cally limited beam rather than a diverging colhmated beam. In the projection system (Fig. 1), a condensing lens forms an image of the light source in the field stop. The projector's objective lens then images the field stop, in air, at the receiver entrance aperture stop. The projector field stop is of such a diameter that its image is the same size as the projector's objective aperture stop. All rays in the illuminated sample path fall within the cylinder defined by the projector aperture and the image of the field stop.
TRANSMISSOMETER PROJECTOR OPTICS
TRANSMISSOMETER RECEIVER OPTICS IMAGE OF PROJECTOR APERTURE
IMAGE OF PROJECTOR APERTURE
IN WATER
Fig 2
The receiver optics are similar (Fig. 2). Behind a window is an aperture, a lens, a field stop, and the detector. Again the lens forms an image of the receiver field stop at the aperture of the projector, but this time in water. This is important to insure that all of the projected flux will get through the system both in air and in water. If, for example, the receiver were designed so that the projector aperture was imaged in the plane of the receiver field stop in air, then when the instrument was submerged this image will fall beyond the field stop and be larger. Some of the rays would then be clipped by the receiver field stop and an erroneous transmission reading will result. The entrance aperture and field stop of the receiver are sized to give the receiver a field of view in water which is a cylinder just slightly larger and encompassing the cylindrical volume illuminated by the projector (Fig. 3).
TRANSMISSOMETER OPTICAL SCHEMATIC
% ERROR • \ • 100 ° M E A -. .TRUE
Fig 1
Fig. 3
Only those rays which appear to be coming from the projector aperture and enter the receiver aperture will arrive at the detector. Ray " A " which may be a ray of scattered light from the projector beam or ambient light, will enter the receiver lens
as if it were coming from outside the projector aperture and will be stopped by the receiver field stop. All rays from the projector are not parallel to the optical axis. The sample volume is traversed by all rays which can be drawn between the area of the projector aperture and receiver aperture. The greatest angle a ray may be deviated and still be accepted by this type of system is indicated by 6, in Fig. 3. This angle is specified by the ratio of the beam diameter to one half the beam length. This upper limit is a special limiting case. The preponderance of scattered light accepted will have a deviation angle much less than this. Preisendorfer (Ref. 1) has shown that for this type of system the percent error, A, will be:
a 2ir a. 100 x- = -100 x
r
~L~ (1)
Detector and Source Considerations
Measured alpha approaches true alpha as the ratio r/L approaches zero. The radiant flux in the beam is also a function of r/L and is given by the expression:
P -n x T -N —- (2)
Where:
P = Radiant flux arriving at the detector T = Transmission of optical system
including filters N = Inherent radiance of the source r = Radius of beam L = Length of beam
Where:
is the forward scattering is the measured volume attenuation coefficient is the true volume attenuation coefficient is the radius of the beam is the length of the beam
Fig. 4 is a curve of error due to forward scattering versus radius to length ratio, from Preisendorfer's paper (Ref. 1) and is based on characteristics of water which is intended to be "Representative of Moderately Clear.Near-Shore Ocean Water". Thus, this type of instrument designed with some care can give good measurements of a for moderately clear oceanic waters. For very turbid waters, the data would be suspect.*
ERROR DUE TO COLLECTION OF FORWARD SCATTERED LIGHT
BASED Oil MODERATELY CLEAR
NEAR SHORE OCEAN WATER
RATIO OF BEAM RADIUS TO LENGTH
Fig. 4
The usable lower limit of the r/L ratio depends upon the inherent brightness of the source and the sensitivity of the detector. Using a very sensitive detector, such as a photomultiplier tube, allows the r/L ratio to be small, and the instrument will give a better a measurement in turbid waters.
For the instrument under discussion an r/L ratio of 1/100 was selected, i.e., 20mm beam diameter and one meter path length. This should provide adequate accuracy in the measurement of alpha in the ocean waters in which it is designed to be used. The chosen r/L ratio provides sufficient flux to allow the use of a solid state detector and avoid the problems of high voltage, temperature stability and fatigue, associated with photomultiplier tubes. Many types of solid state photovoltaic and photo-conductive detectors were tested for sensitivity, temperature coefficient, stability, and fatigue characteristics. A silicon photovoltaic cell was chosen. A typical silicon detector has the spectral response shown in Fig. 5. It is responsive to flux in the region roughly from 350 to 1100 nanometers, with its peak response at 850 nanometers in the near infrared. This red sensitivity combined with the red-rich radiant energy from the tungsten source make it imperative that an infrared rejection filter be inserted in the optical system. Otherwise, one would be making an a measurement heavily weighted by the flux in the red and infrared region where the transmission is very low and of little interest. A Shott BG-18 glass filter was selected for this purpose. (Another excellent filter for this purpose
A program is currently under way to study the magnitude of this error empirically for various optical designs in various types of water.
3
SPECTRAL RESPONSE OF SILICON PHOTOVOLTAIC CELL
TRANSMISSOMETER OPTICAL SCHEMATIC
400 000 800 1000
WAVELENGTH NANOMETERS
Fig. 5
is the Corning glass filter, color specification 4-76.) Further restriction of the spectral band width can be obtained through the use of Wratten gelatin filters. The IR rejection filter is still necessary since Wratten filters transmit freely in the near infrared. Using a Wratten 64 filter gives the instrument the overall sensitivity shown in Fig. 6, which falls in the region of maximum transmission for most natural waters.
The source is a 20W tungsten-iodide lamp selected for its stability and filament size and shape.
SPECTRAL SENSITIVITY OF TRANSMISSOMETER
RELATIVE SENSITIVITY
\ / 'N^TRANSMISSOMET.ER
SO . \l 1 BAY
- 80 1 COASTAL \ s
e 0 40
•
20
•
\ y^OISTILLEO _ ^ j ( WATER
°4 DO 500 600 7 0
Fig. 6.
WAVELENGTH - NANOMETERS
Relative Sensitivity of Transmissometer with Tungsten Source, Silicon Detector and Shott BG-18 and Wratten 64 Color Filters.
Instrument Description
A schematic drawing of the instrument is presented in Fig. 7. A porro prism was used to fold the water path. This shortens the instrument making it more convenient to handle and puts the projector, receiver, and all electrical connections in one housing. More importantly, it places the projector and receiver in close proximity and it becomes a simple matter to have a light path directly from the lamp to the detector to obtain a "closed
Fig. 7
loop" optical system. A light pipe runs from the lamp to the photocell and a shutter device enables the cell to receive light through the water path, through the internal reference path, or placed in the dark to check for dark current or leakage currents. With this arrangement the operator can check overall system sensitivity during the data taking process. With a sensitivity adjustment he can return the reference reading to its proper value and thus adjust out any changes in lamp output or cell sensitivity due to temperature or fatigue.
The shutter is driven through a geneva mechanism by a small D.C. motor running on a flashlight battery. The motor is held connected to the battery for one geneva cycle by a latching relay which draws its power from the voltage drop in the lamp leads in the cable and latches upon command from the operator. Fig. 8 is a schematic of the transmission measurement circuit. The lamp is operated from a highly regulated power supply operating in the current regulated mode. By regulating the current the changes in the voltage drop in the cable do not affect the output from the lamp.
The semiconductor photovoltaic cell must work into a low impedance in order to obtain linearity of output current with flux and have a low temperature coefficient. The output of the photocell is less than 20/xa and is amplified by a small solid state
TRANSMISSOMETER CIRCUIT
CALIBRATE ADJUSTMENT
NULL METER
INTERNAL I I REFERENCE PATH
LLdP— • - ] * ( C ) LAMP
I I
I READOUT POTENTIOMETER
/ T' Li Is
READOUT-'
VOLTAGE SOURCE
I I
~\ h
CURRENT REGULATED _ l _ ADJUSTABIE - = .
POWER SUPPLY T
UNDERWATER HOUSING
Fig. 8
4
operational amplifier connected to have an input
impedance of ten ohms and to provide a full s ca l e
output of up to 10 vo l t s . The signal from a strain
gage type pressure t ransducer is amplified by a
similar operational amplifier to obtain 40 mV/meter
Signal. Both s igna ls go to the wipers o f t en turn
potentiometers by means of which they are compar
ed Id a reference voltage in the surface control unit.
The potentiometer d ia l s indicate beam transmission
and depth direct ly. Alternatively the output from
the two channels can be applied directly to an X-Y
plotter to obtain a depth profile of t ransmission per
meter direct ly.
The air reading for this instrument is se t to be
85.5%. Th is accounts for the reflection loss on the
four window interfaces being greater in air than
water. The internal reference path is se t to read
some arbitrary value c lose to 100%.
KIR. i). Transmissometer with Control Unit .
A photograph of the instrument and its control
unit is shown in F ig . 9, and the instrument with the
watertight cover removed is shown in F ig . 10. The
transmissometer alone is thirty-five inches long and
weighs thirty-five pounds in air.
Summary
All t ransmissometers allow scat tered light to
reach the detector in varying degrees . The t rans
missometer which has been d i s cus sed confines the
F i g . 10 . Transmis someter with C o v e r H.MIIOV.MI.
beam from the projector to a cylindrical volume and
limits the field of view of the receiver to a cylinder
c losely matching the illuminated volume. Th i s re
duces the error, due to forward sca t te r ing . Th i s
error in a cyl indrical ly limited beam transmissometer
is a function of the forward sca t te r ing property of
the water and the beam radius to length rat io . The
lower limit of the radius to length ratio which may
be used is determined by the radiance of the source
and the sens i t iv i ty of the de tec tor . The optics for
this type of system must be designed carefully so
that al l of the flux p a s s e s through the system in
both air and water to enable a reading in air to be
used a s a cal ibrat ion check. The instrument de
scribed has an internal reference light path to allow
a check of the s tabi l i ty of the cal ibrat ion, and a
sensi t ivi ty adjustment permits correct ions to be
made, if n e c e s s a r y , while the instrument is sub
merged. It i s designed to be a readily usab le ,
rel iable instrument for ocean survey work and is
sufficiently accurate for this purpose.
Acknowledgments
R. W. Loudermilk, A s s o c . Engineer with the
Visibil i ty Laboratory, contributed much to this
effort and supervised the design and fabrication.
References
1. A General Theory of Perturbed Light F i e l d s ,
with appl ica t ions to Forward Scatter ing Effects
in Beam Transmit tance Measurements by R.W.
Preisendorfer , 1 May 1958, Index No. NS 714-100,
SIO Ref. 58-37.
5
UNCLASSIFIED Security Classification
DOCUMENT CONTROL DATA • R&D (Security claealllcatlon ol title, body ol abatract and Indexing, annotation muat be entered when the overall report le claaaltiad)
t. ORIGINATING ACTIVITY (Corporate author)
Visibility Laboratory University of California San Diego, California 92152
2a. REPORT SECURITY CLASSIFICATION
UNCLASSIFIED 2b GROUP
S. REPORT TITLE
AN UNDERWATER TRANSMISSOMETER FOR OCEAN SURVEY WORK
4. DESCRIPTIVE NOTES (Type ol report and Inctuelve datea)
Technical Report B. AUTHORfS) (Laatname. IIret name, Initial)
Petzold, Theodore J. and Austin, Roswell W.
6. REPORT DATE
April 1968 7a. TOTAL NO. OP PAGES 76. NO. OF REPS
Sa. CONTRACT OR GRANT NO.
NObsr-95251 b. PROJECT NO.
SF0180201, Task 538 O.
Task I d.
9a. ORIGINATOR'S REPORT NUMBBRC5)
SIO Ref. 6 8-9
9b. OTHER REPORT NOf5) (A ny other number* that may be aaaltned (hie report)
10. AVAILABIL ITY/L IMITATION NOTICES
Distribution of this document is unlimited.
I I . SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY
Naval Ship Systems Command Department of the Navy Washington, D. C. 20360
IS. ABSTRACT
A transmissometer developed at the Visibility Laboratory, University of California at San Diego is described. This instrument is capable of being operated by nontechnical persons with little instruction. No compromise has been made in the precision of measurement to obtain the design objectives which include ease of operation, low maintenance, and reliability. The optical system, which has a cylindrically limited beam rather than a collimated source of light, is discussed.
DD MS*.. 1473 0101-807-6800 UNCLASSIFIED Security Classification
UNCLASSIFIED Security Classification
KEY WORDS ROLE W ROLE WT
Underwater Optics Water transparency Alphameter Transmissometer Water optical attenuation
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